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8-104 (4) and (5)

Why do we have to derate the overcurrent device MORE with single conductor? With a 100% continuous rated breaker we can rate the circuit at 100 % with multiple conductors but only at 80% with single conductor.

Can anyone shed some light on why it would be lees when a single conductor is rated for more amps?

Same for subrule 5 where it is am 80% rated breaker and we use 70% for single conductor and 80% multiconductor.
 

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I'd have to dig out code, but I think it's because table 1&3 give you higher ampacities for the same awg of wire as table 2&4.

So that higher current gives more heat and needs to be derated?

Depending on the situation, even though you could use table 1 or 3, maybe it's beneficial to use table 2 or 4... To get the 100%.

Just guessing here.
 
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